Magnetic resonance imaging (MRI) is a medical imaging technique that applies a magnetic field and a pulse of radio frequency (RF) energy to produce images used for characterizing internal biological structures. MRI is based on the property of nuclear magnetic resonance (NMR). NMR is a physical property in which the nuclei of atoms absorb and re-emit electromagnetic energy at a specific resonance frequency in the presence of a magnetic field. The absorption and reemission of energy can be dependent on the strength of the magnetic field and the magnetic property of the atoms (e.g., atoms whose nuclei possesses magnetic spin).
MRI offers the ability to produce various types of contrast within an image, e.g., which can be referred to as ‘weighting’. For example, water protons and other atomic nuclei have characteristic NMR properties based on their physical and chemical environments that can be used to create a contrast effect between different types of body tissue or between other properties. Various MRI weighting techniques can be employed to differentiate regions of interest within biological tissue based on these differences in these NMR properties.
For example, magnetic resonance imaging can be performed in the following way. For example, a strong static magnetic field can be generated through a region of a subject's body that aligns the average magnetic moment of protons (e.g., water protons within the tissues of the body) with the direction of the applied magnetic field. A dynamic electromagnetic field (e.g., an RF pulse), which includes the resonance frequency of the protons, can be generated such that the dynamic electromagnetic field is absorbed and flips the spin of the protons in the magnetic field. For example, between RF pulses or after the electromagnetic field is turned off, the proton spins return to their equilibrium state realigning the bulk magnetization with the magnetic field, thereby producing a measurable radio frequency signal. During this relaxation state, the proton spins return to their equilibrium at different rates, e.g., based on the different physical and chemical environments of the protons. Signal processing techniques can be implemented that determine various parameters (e.g., spin density, T1 and T2 relaxation times, and flow and spectral shifts) used to produce an MRI image. In addition, for example, MRI can include the generation of additional magnetic fields during the RF signal scan to obtain three dimensional information of a target tissue.